Method of using an array of electrodes for high throughput development and testing of materials

ABSTRACT

Methods and apparatus employ the use of arrays ( 301, 902 ) of two or more electronically-discrete electrodes ( 1, 2, 3  . . . ) to facilitate high-throughput preparation and testing of materials comprising two or more elements. High rates of deposition, synthesis and/or analysis of materials are achieved with the use of arrays of electrodes whereby desired materials are developed. The high rate synthesis and/or analysis of an array of materials uses deposition control techniques in conjunction with the electrode array to develop a meaningful array of materials and to analyze the materials for desired characteristics to develop one or more materials with desired characteristics. The use of an array of electrodes enables high-throughput development of materials having scientific and economic advantages.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 09/869,102 Jun. 22, 2001U.S. Pat. No. 6,683,446 which is a 317 of PCT/US99/30812 Dec. 21, 1999which claims benefit of provisional 60/113,162 Dec. 22, 1998.

The United States Government has certain rights in this inventionpursuant to Contract No. N00014-99-1-0354 between the U.S. Office ofNaval Research and the University of Wyoming.

FIELD OF THE INVENTION

The present invention generally relates to methods and apparatus for thehigh rate deposition, synthesis and/or analysis of materials on an arrayof electrodes, and the desired materials developed from the methods.More specifically, the invention is directed to methods of high ratesynthesis and/or analysis of an array of materials wherein depositioncontrol techniques in conjunction with the electrode array are employedto develop a meaningful array of materials and to analyze the materialsfor a desired characteristic to develop one or more materials with thedesired characteristics.

BACKGROUND OF THE INVENTION

Developing new and useful materials, in the past, has been by predictionof the general chemistry of compositions and applying known testingmethods to a small number of synthesized materials. Even with predictingand applying the currently known chemistry of materials, the number ofmaterials that are predicted in a group is too large to properlyanalyze. The result of only analyzing a few materials in a predictedgroup leaves the great majority of predicted useful materialsunexplored. Thus, the discovery and development of new materials haveneed for a method of synthesizing and analyzing new materials with alarge number of variable compositions at a high throughput rate.

Combinatorial methods represent a new set of experimental tools that arewell suited to explore systems comprised of a very large number ofvariable compositions. As a consequence of this characteristic, therehas been a great deal of recent activity in the application ofcombinatorial synthesis to drug discovery, Chem. Rev. 1997, 97(2), wheresuch large number of variable compositions are commonplace. In casessuch as these, a great many different chemical structures need to beexamined to find structural motifs, amino acid sequences (e.g. inbioactive polypeptides), or other molecular characteristics that exhibitthe desired effect. The key to success in these efforts has been toexploit the power of combinatorial methods both for doing chemicalreactions and for examining the efficacy of the resulting compounds, allin a parallel or high-speed serial fashion. The range of types ofsynthetic schemes and the systems to which they have been applied istypified by the articles in Chemical Reviews theme issue (Chem. Rev.1997, 97(2)).

More recently, several groups have begun to apply combinatorial methodsto materials problems. An example of this trend is the work being doneat UC Berkeley by Schultz et al. To date, these groups have focusedpredominantly on materials properties, especially luminescence. Also, arecent report by Mallouk et al. points to the use of such methods inelectrochemical applications (Science, 1998, 280, 1735). Specifically,Mallouk et al. used ink jet processing to deliver multiple metalcomplexes that served as electrocatalyst precursors to specific sites ona conductive substrate, employed chemical methods to reduce thecomplexes to produce metallic alloys and then used a novelfluorescence-based method to look for methanol oxidation activity. Thisappears to be one of the first uses of combinatorial methods indevelopment of electrocatalysts. A particularly useful feature of thismethod was the demonstration of a parallel testing method. In additionto the efforts described above, several other groups have begun toexplore the use of combinatorial methods for synthesis of materials withnovel properties (Briceno et al. Science, 1995, 270, 273; Kobayashi etal. J. Am. Chem. Soc. 1996, 118, 8977).

An electrode's oxidation and reduction capabilities have led to the useof electrodes performing an essential step in synthesizing materials.One of the earliest description of using electrodes in combinatorialsynthesis is by Fodor et al. (U.S. Pat. No. 5,424,186). Microelectrodesare used to remove protecting groups in the synthesis of organicmolecules. Fodor et al. position an electrode over the protecting groupto activate the desired deprotection step. Because of an electrode'sversatility and control, the use of an array of electrodes in synthesisand analysis of materials is forthcoming.

In depositing materials onto an electrode many factors contribute to thecomposition of the material in the array. Some factors even affect thedeposited materials in a solution after the material has already beendeposited and other compositions are being deposited. In WO98/03521,Weinberg et al. express the need for homogeneous compositions ofmaterials for a meaningful analysis of an array of materials. However,little work has been done to ensure that the array of materials may beanalyzed for a desired characteristic and not for unwanted variations inmorphology or composition.

An important feature of combinatorial synthesis is the ability todeposit meaningful compositions at discrete electrodes at a high rate ofspeed. In PCT WO98/14641, the complete disclosure of which isincorporated herein by reference for all purposes, McFarland et al. showan array of electrodes used for combinatorial synthesis and analysis,however, the use of changing out or adding components of the solutionsin a solution bath results in an increased number of solutions when ahundred compositions are synthesized. Additionally, when more electrodesare employed to synthesize thousands or ten of thousands ofcompositions, the number of solutions or additions to solutions neededadversely affects the ability of high-throughput synthesis ofcompositions. McFarland et al., in WO98/14641, attempt to alleviate theneed for a high number of solutions or additions to solution by using avariety of potentials at different electrodes to attempt to adjust thedeposition of certain components in the solutions to vary thecompositions at the electrodes electronically. While this method mayresult in a desired library of compositions, the compositions areaffected by the method used to deposit and any meaningful analysis orscreening is adversely affected by the morphology of the compositions.McFarland et al. discuss how multiple samples of varying composition canbe prepared from solutions carrying various metal salts. However, theydo not take the necessary steps to produce controllable morphologyduring the deposition or to maintain the composition of the samplesafter the deposition. For instance, when electrodepositing metals fromsolution at overpotentials that vary, a wide variety of surfacemorphologies are created. Those surface morphologies preclude easy andrapid comparison of the physical or chemical properties of the samples,specifically of the electrochemical, catalytic, or optical properties.Furthermore, when electroplating from solutions that contain Ni, Fe, Cu,and Zn, deposited samples that contain Zn, Ni, or Fe at their surfaces,the Zn, Ni, or Fe will react with the solution-bound salt of Cu todissolve Zn, Ni, or Fe and deposit Cu. Similar reactions occur betweenZn and Ni and between Zn and Fe. This general type of reaction occursbetween any two species where the redox state of one species is at aless positive potential than the redox species of another species in thesame environment. These reactions inadvertently change the surfacecompositions and morphologies of the deposits that have been preparedbut remain in contact with the precursor-containing solution. Thus,controllable high-throughput synthesis and analysis of new materialsusing an array of electrodes is not yet feasible.

In order to synthesize and analyze a large number of new materials, amethod of developing and analyzing new materials on an array ofelectrodes employing control techniques to ensure desired compositionsand morphologies at known locations on the array is desirable.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for the high ratedeposition, synthesis and/or analysis of materials of variouscompositions onto an array of electrodes, and the materials developedfrom the methods. In particular, the present invention provides methodsof high rate synthesis and/or analysis of an array of materials whereindeposition control techniques in conjunction with the electrode arrayare employed to develop a meaningful array of materials wherein thearray of materials may be analyzed for desired characteristics todevelop one or more materials with the desired characteristics.

Array of Electrodes

In order to synthesize a large number of materials with varyingcompositions, an array of electrodes is employed. The array allows thecontrol necessary for high-throughput synthesis of new materials. Thearray uses a conducting material to contact two or more discreteconducting regions to produce two or more electronically-discreteelectrodes. The array has two or more electronically-discrete electrodeswhich are addressable individually or collectively, in serial or inparallel, using electronic, optical, or mechanical means, via passive oractive, internal or external circuitry. The use of the array ofelectrodes is preferably by addressing the discrete electrodesindividually AND collectively. The electrodes provide an electricalpotential or electrical current to initiate deposition of a desiredcomposition at the electrode. The electrodes are controlled by meansthat allow a predetermined composition to be deposited at a knownelectrode. Thus, for any given electrode the composition of the materialdeposited at that electrode is known when the entire array of materialshas been synthesized. The electrode array consists of two or moreelectronically-discrete electrodes, preferably of twenty or moreelectronically-discrete electrodes, or, more preferably, of 100 or moreelectronically-discrete electrodes, or, more preferably, of 1000 or moreelectronically-discrete electrodes, or, more preferably, of 10,000 ormore electronically-discrete electrodes, or, most preferably, of 100,000or more electronically-discrete electrodes.

Deposition

The deposition of materials onto the array of electrodes may be byelectrodeposition or co-electrodeposition of one of more elements viareductive or oxidative passage of one or more electrons between theelectrodes comprising the array or from some external electrode assemblyand the elements, assembly of elements, or chemical or physicalassemblies containing the element or elements; electrophoreticdeposition of one or more elements via electrostatic interaction betweenthe elements, assembly of elements, or chemical or physical assembliescontaining the element or elements and the electrodes comprising thearray or from some external electrode assembly; electrochemically-,chemically-, or physically- induced deposition or precipitation ofelements, assembly of elements, or chemical or physical assembliescontaining the element or elements; or spontaneous precipitation ofelements, assembly of elements, or chemical or physical assembliescontaining the element or elements.

Deposition may include the introduction of the electrode array into asolution or mixture of components for deposition. Alternatively, thesolution or mixture may be introduced to the electrode array. Thesolution or mixture entrains the components for deposition and suppliesthe components for deposition onto the electrode when the electrode isaddressed in some fashion. The variation of the deposition components inthe solution or mixture is controlled to allow a known composition to bedeposited at a known electrode. The controlled variation of thedeposition components may be achieved by any method wherein the knowndeposition components are present at the known electrode to deposit theknown composition at the electrode.

Deposition may also employ the use of a counter-electrode and referenceelectrode or simply a counter electrode. The counter-electrode providescurrent to complete the circuit through the cell. The referenceelectrode provides for control of the potential applied at the electrodein the array of electrodes.

Control of Deposition

In order to ensure that the array of materials may be meaningfullyanalyzed, various deposition control techniques are employed. Thedeposition of materials onto an array of electrodes may lead to varyingmorphologies amongst the varying materials on the array. One discretematerial deposited at one discrete electrode may have an extremely roughsurface or morphology compared with the other materials deposited atother electrodes. With some deposition techniques, the morphologies mayvary material to material. When analyzing the materials, some desiredcharacteristics are affected by the morphology, and the ability tocontrol morphologies along with other deposition characteristics ishighly critical to the analysis of the materials for desiredcharacteristics.

Alternately, the ability to deliberately generate a wide variety ofmorphologies is highly desirable when such morphologies comprise desiredcharacteristics of the deposits.

The control techniques include methods to adjust or control themorphologies of the depositing or deposited materials, methods toprotect the deposited materials from further reactions, methods tocontrol the potential at the electrodes where deposition has occurred,methods to control the exchange of reactive species at depositedmaterials, methods to control the potential at the depositing material,methods to cap or passivate the deposited material, methods to controlthe current at the electrodes, methods to control the counter-electrodeor reference position and other methods to deposit a homogeneousmaterial at discrete electrodes. The control techniques and methods arenot limited to the deposition process and may include methods to controlor adjust the materials after deposition of all materials on the array.These methods may include additional steps before analysis wherein thematerials are processed further to ensure a homogeneous composition ateach electrode or one or more homogeneous characteristic at eachdiscrete composition location that is suitable for analysis of thedesired characteristic sought.

The methods employed to control the deposition of the materials onto theelectrode array include but are not limited to pulse electrodeposition,potential control to avoid exchange reactions, overpotentialelectrodeposition, the use of kinetically sluggish precursors, thepositioning of one or more counter-electrodes or reference electrodesand the use of passivating layers.

Processing

After deposition of the materials on the electrode array an additionalstep may be employed to further process the materials for analysis. Theprocessing step may occur while the material array is still withincontact of the solutions or other components of the deposition step.Processing may include exposure of the array of materials to gaseous,liquid, or solid reactants, controlled heating or cooling of the arrayof materials, and treatment of the array of materials withelectromagnetic radiation of wavelength between 10⁻¹⁶ m and 10⁻⁸ m.

Analysis

This invention also relates the use of arrays of electrodes as describedto analyze materials comprising two or more elements. The array ofelectrodes may be used to synthesize the array of materials, but thearray need not be used to synthesize the materials. Preferably, thearray of electrodes is used to synthesize and analyze the array ofmaterials. Methods used to analyze the array of materials may or may notcomprise combinations of one or more methods including, but not limitedto electrochemical analysis of materials via the electrodes contained inthe array or via some external electrode assembly. Other analysistechniques include electrochemical analysis of the materials usingelectrochemical methods including but not limited to potentiometry,coulometry, voltammetry, and polarography; analysis of the materials viaoptical methods including but not limited to infrared, Raman, electronicabsorption, fluorescence, phosphorescence, and chemiluminescencespectroscopies, atomic spectroscopy, emission spectroscopy based onplasma, arc, and spark atomization, nephelometry, turbidity,refractometry, polarimetry, rotatory dispersion, and circular dichroism;analysis of the materials via x-ray spectroscopies including, but notlimited to, x-ray fluorescence, absorption and diffractionspectroscopies; analysis of the materials via electron spectroscopicmethods including, but not limited to, x-ray photoelectron spectroscopy,ultraviolet photoelectric spectroscopy, Auger spectroscopy, ionneutralization spectroscopy, electron impact spectroscopy, and penningionization spectroscopy; analysis of the materials via nuclear magneticresonance methods including, but not limited to, nuclear magneticresonance spectroscopy and electron spin resonance spectroscopy; andanalysis of the materials via other methods including, but not limitedto, radiochemical methods, mass spectrometry, conductometric methods,thermal methods, and chromatographic separations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for high throughput combinatorial synthesisof materials;

FIG. 2 illustrates a cross section of an electrode array;

FIG. 3 illustrates an array of addressable electrodes wherein 99different compositions are obtained during high-throughput combinatorialsynthesis;

FIG. 4 illustrates a control technique of pulse electrodeposition;

FIG. 5 illustrates two potentials in deosition;

FIG. 5 a illustrates the effects of varying the potential whiledepositing;

FIG. 5 b illustrates controlled deposition using overpotential;

FIG. 6 a illustrates the process of exchange reactions;

FIG. 6 b illustrates the result of an exchange reaction;

FIG. 7 illustrates the control of potential to reduce exchangereactions;

FIGS. 8 a-f illustrate the use of passivating layers;

FIG. 9 illustrates the positioning of a counter-electrode and areference electrode;

FIG. 10 illustrates the effect of controlling deposition by the methodillustrated in FIG. 9;

FIG. 11 illustrates an overview of the positioning of acounter-electrode and a reference electrode;

FIG. 12 illustrates an overview of the effect of controlling depositionby the method illustrated in FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Using arrays of electrodes to synthesize a large number of materials hasbeen shown. Thus far, high rates of synthesis and analysis have not beenreached because of the arrays, deposition methods and other factors. Inorder to exploit the use of large arrays of electrodes in developingmaterials, control techniques are needed to assist high rates ofdevelopment and analysis. Employing an array of electrodes withoutappropriate control of deposition and material properties adverselyaffects any analysis conducted on the material array. The result ofcontrol techniques in developing an array of materials using electrodesis a meaningful array of materials which may be productively analyzedfor desired characteristics. By employing control techniques along withthe use of the following electrode arrays, deposition steps, processingand analysis, a high rate or high-throughput method and apparatus ofsynthesizing and analyzing an array of materials is disclosed.

The essential elements needed for the high-throughput synthesis andanalysis of new materials are depicted in FIG. 1. The system depicted inFIG. 1 includes a controller 101 that is programmed for operation eithermanually or by computer 102. The controller 101 is connected via wires103 to the working electrode array 104, the reference electrode(s) 105,and the counter electrode(s) 106. The array of electrodes (the “working”electrode array) 104 is immersed in a container or flow cell 107 throughwhich liquid flows. The liquid contains precursors to electrodepositionand, in some cases, supporting electrolyte salt. The precursors arestored in containers 111. The liquid enters the flow cell 107 throughone or more inlet holes 108 and leaves the flow cell 107 through one ormore outlet holes 109. The solution is delivered to the flow cell 107 bypressure from a mechanical or other type of pump (not shown). The pumpdelivers the solution directly to the flow cell 107 or delivers thesolution to a mixing chamber 110 which then is pressurized so thatsolution flows to the flow cell.

Array of Electrodes

In order to synthesize a large number of materials with varyingcompositions, an array of electrodes is employed. The array allows thecontrol necessary for high-throughput synthesis of new materials. Thearray uses a conducting material to contact two or more discreteconducting regions to produce two or more electronically-discreteelectrodes. The array has two or more electronically-discrete electrodeswhich are addressable individually or collectively, in serial or inparallel, using electronic, optical, or mechanical means, via passive oractive, internal or external circuitry. The use of the array ofelectrodes is preferably by addressing the discrete electrodesindividually AND collectively. The electrodes provide an electricalpotential or electrical current to initiate deposition of a desiredcomposition at the electrode. The electrodes are controlled by meansthat allow a predetermined composition to be deposited at a knownelectrode. Thus, for any given electrode the composition of the materialdeposited at that electrode is known when the entire array of materialshas been synthesized. The electrode array consists of two or moreelectronically-discrete electrodes, preferably of twenty or moreelectronically-discrete electrodes, or, more preferably, of 100 or moreelectronically-discrete electrodes, or, more preferably, of 1000 or moreelectronically-discrete electrodes, or, more preferably, of 10,000 ormore electronically-discrete electrodes, or, most preferably, of 100,000or more electronically-discrete electrodes.

A cross section of a typical array used in the high-throughput synthesisand analysis of new materials is shown in FIG. 2. FIG. 2 depicts ahighly resistive substrate 201 upon which is located highly conductiveelectrodes 202. The electrodes are spatially separated to beelectronically discrete. Connecting each electrode to the arraycontroller is a highly conductive wire 203. The array controller appliespotential and/or current conditions to each highly conductive wire 203and thus to each highly conductive electrode 202. A highly resistivematerial 204 is coated on top of or around the wires in order to avoidredox reaction between the wires and the solution. The highly resistivematerial 204 does not cover the surfaces of the electrodes 202.

An electrode array is composed of discrete highly conductive electrodes,preferably disk-shaped, placed on top of a highly resistive substrate orembedded into the surface of a highly resistive substrate such that thehighly conductive electrodes are physically separated from each other.In some cases, the electrodes are connected to an external multiplexingunit such that each electrode can be individually addressedelectronically. In other cases, the electrodes are connected to commonwires that carry voltage and current signals from the controllingpotentiostat to the electrodes. In the latter case, the common wires cannumber fewer than the electrodes or number the same as the electrodes ornumber more than the electrodes. Connection of the electrodes to thewires can occur by switched contacts or by permanent contacts. In allcases, the conductive portions of the array of electrodes are completelycovered by either an over-coated highly resistive layer or by highlyresistive materials that are part of the components, with the exceptionof the electrode faces, which remain uncovered.

In the preferred embodiment an analog, very large scale integratedcircuit (VLSI), complementary metal oxide semiconductor (CMOS),microelectrode array (MEA) chip is used. The MEA is comprised of atleast 1,024 microelectrodes that can be addressed individually orgrouped, in series or in parallel, and can be used in all of thetraditional electrochemical synthetic motifs. This unique platform canbe used with high-throughput electrodeposition methodologies that allowproduction of very large numbers of materials in a very short time.

The MEA electrodes are deposited using traditional silicon fabricationtechniques. As such, each microelectrode (measuring less than 100 μm indiameter) is built on an Al pad deposited on the silicon wafer. Afterdeposition of the typical W/Ti Al diffusion barrier, an overcoat of thedesired electrode material is applied, to give functionalmicroelectrodes.

The electrodes may be smaller than 10 cm², or smaller than 10 μm², oreven smaller than 1 μm².

The electrodes can be coupled to single or multiple power supplies, oralternately they can be placed in series with current or power limitingdevices (inductors, resistors, etc.). In a preferred embodiment, thearray of electrodes can be controlled as to potential and/or current bymaintaining multiple wires at multiple potentials and/or currents. Theelectrodes can then, singly, in groups, or in total, be reversiblyswitched to the appropriate wire to place the electrode under thedesired potential and/or current condition. The switches used to performthis reversible switching operation can include mechanical, electrical,optical, or magnetic materials. For instance, transistors, CMOS, throwswitches, etc. can be used for switches. The critical issue is that thepotential and/or current condition on each wire is not changed with timeduring the experiment. Only the electronic circuitry of the system ischanged to move electrodes from one wire to another wire. Otherembodiments of this controlling and addressing circuitry will be obviousto those skilled in the art.

Deposition

The deposition of materials onto the array of electrodes may be byelectrodeposition or co-electrodeposition of one of more elements viareductive or oxidative passage of one or more electrons between theelectrodes comprising the array or from some external electrode assemblyand the elements, assembly of elements, or chemical or physicalassemblies containing the element or elements; electrophoreticdeposition of one or more elements via electrostatic interaction betweenthe elements, assembly of elements, or chemical or physical assembliescontaining the element or elements and the electrodes comprising thearray or from some external electrode assembly; electrochemically-,chemically-, or physically-induced deposition or precipitation ofelements, assembly of elements, or chemical or physical assembliescontaining the element or elements; or spontaneous precipitation ofelements, assembly of elements, or chemical or physical assembliescontaining the element or elements.

Deposition may include the introduction of the electrode array into asolution or mixture of components for deposition. Alternatively, thesolution or mixture may be introduced to the electrode array. Thesolution or mixture entrains the components for deposition and suppliesthe components for deposition onto the electrode when the electrode isaddressed in some fashion. The variation of the deposition components inthe solution or mixture is controlled to allow a known composition to bedeposited at a known electrode. The controlled variation of thedeposition components may be achieved by any method wherein the knowndeposition components are present at the known electrode to deposit theknown composition at the electrode.

Deposition may also employ the use of a counter-electrode and referenceelectrode or simply a counter electrode. The counter-electrode providescurrent to complete the circuit through the cell. The referenceelectrode provides for control of the potential applied at the electrodein the array of electrodes.

The spatially-heterogeneous chemical concentration of the precursor inthe solution can be varied by varying the input feedstock position andconcentration. Thus, in a heterogeneous solution of precursor, an arrayof electrodes which all electrodeposit at the same time will depositdifferent materials. In a preferred embodiment, shown in FIG. 3, flowinga solution of regularly (time-based) varying composition (e.g. from 1%X, 99% Y, to 2% X, 98% Y . . . to 99% X, 1%, Y, where X and Y representdifferent deposition precursors) across an array of electrodes anddepositing at a single electrode at a time quickly prepares samples ofregularly-varying composition without having to remove the solution orany substantial part of the solution. While the composition of theflowing solution varies slightly over time, at no time does the flowingsolution vary significantly across the surface of the array. Forexample, an array can be placed in a flow cell containing a referenceelectrode, a counter electrode, and inlet and outlet ports for liquidsolution. A deoxygenated 1 liter solution of 1 mM CuSO₄ is flowed,during the process, into a mixing chamber containing 1 liter of 1 mMAgNO₃. The mixed solution is flowed into the flow cell during theprocess. Early in the process, the flow cell contains solution thatcontains primarily AgNO₃. Late in the process, the flow cell containssolution that contains primarily CuSO₄. Between these extremes, thesolution composition changes gradually from mostly AgNO₃ to mostlyCuSO₄. During this change in solution composition, the electrodes in thearray of electrodes are biased individually and discretely to one ormore potentials that cause deposition of the CuSO₄ and/or AgNO₃. Afterdeposition at one electrode, that electrode is returned to a neutralpotential and another electrode is turned “on” at a depositingpotential. This “serial deposition” is continued until all electrodeshave been so treated. The result of this experiment is an array ofdeposited samples of regularly-varying composition suitable forcollection, treatment, and/or analysis.

Control of Deposition

In order to ensure the array of materials may be meaningfully analyzed,various deposition control techniques are employed. The deposition ofmaterials onto an array of electrodes may lead to varying morphologiesamongst the varying materials on the array. One discrete materialdeposited at one discrete electrode may have an extremely rough surfaceor morphology compared with the other materials deposited at otherelectrodes. With some deposition techniques, the morphologies may varymaterial to material. When analyzing the materials, some desiredcharacteristics are affected by the morphology, and the ability tocontrol morphologies along with other deposition characteristics ishighly critical to the analysis of the materials for desiredcharacteristics.

Alternately, the ability to deliberately generate a wide variety ofmorphologies is highly desirable when such morphologies comprise desiredcharacteristics of the deposits.

The control techniques include methods to adjust or control themorphologies of the depositing or deposited materials, methods toprotect the deposited materials from further reactions, methods tocontrol the potential at the electrodes where deposition has occurred,methods to control the exchange of reactive species at depositedmaterials, methods to control the potential at the depositing material,methods to cap or passivate the deposited material, methods to controlthe current at the electrodes, methods to control the counter-electrodeand other methods to deposit a homogeneous material at discreteelectrodes. The control techniques and methods are not limited to thedeposition process and may include methods to control or adjust thematerials after deposition of all materials on the array. These methodsmay include additional steps before analysis wherein the materials areprocessed further to ensure a homogeneous composition at each electrodeor one or more homogeneous characteristic at each discrete compositionlocation that is suitable for analysis of the desired characteristicsought.

The methods employed to control the deposition of the materials onto theelectrode array include but are not limited to pulse electrodeposition,potential control to avoid exchange reactions, overpotentialelectrodeposition, the use of kinetically sluggish precursors, thepositioning of one or more counter-electrodes and the use of passivatinglayers.

Controlling the electronic state of the reference electrode can act tovary the potential at different electrodes, which can act to vary thecomposition and/or thickness of deposits at those electrodes.Additionally, varying the physical location of the counter electrode canvary the real current density at each electrode surface for a number ofelectrodes that are held at the same total and apparent current density.For example, as shown in FIGS. 9-12, for an array of electrodes 902where every electrode 901 is held at a potential that can initiate oneor more redox events with solution species at rates determined by theindividual current densities across each electrode 901, said currentdensities can be varied across the array of electrodes 902 merely bypositioning the counter electrode 903 closer to one part of the arraythan to other parts of the array. Varying said current densities in thismanner results in making thicker deposits 904 at the electrodes 901nearer the counter electrode 903 compared to thinner deposits 905 madeat the electrodes 901 farther from the counter electrode. This providesa ready mechanism to control the gross amount of materialelectrodeposited onto a particular electrode. Depositing other sampleson top of said material then allows both compositional and structuralcontrol over the total sample prepared at a particular electrode.

Pulse electrodeposition techniques can be used in combination witharrays of electrodes to prepare libraries of samples that have similaror nearly identical surface morphologies. For example, an array ofelectrodes can be placed into a solution containing electrodepositionprecursors. One or more electrodes can then be placed under potentialcontrol in such a way that the electrodeposition precursors are reactedto form deposits on the electrodes. Such reaction can occur by reducingthe Fermi level (or “potential”) of the electrodes until electrons passfrom the electrodes to the precursors, where that electron transfer isaccompanied by deposition of the desired composition.

Typically, such “reductive deposition” as described above results indeposits with widely varying morphologies that depend on precursorconcentration, movement of the solution, temperature, and potential.Such variations in morphology introduce considerable difficulty whencomparing the physical and chemical properties of the deposits.

In the present invention, the morphologies of the deposits that areprepared onto the array of electrodes may be controlled to be similar ordissimilar by using pulse electrodeposition. Pulse electrodepositionmethods involve pulsing the potential to regions where the precursorsare reduced, followed by returning to a potential region where littlereaction occurs. FIG. 4 shows the potential vs time plot typical of apulse electrodeposition synthesis process cycle. The starting potential401, pulse potential 402, pulse duration 403 and rest time 404 betweenpulses can be controlled to provide the morphology required.

For example, an array of 1024 electrodes that are individuallycontrolled and held at +0.5 volts (V) (vs. the standard electrochemicalreference redox couple of Ag/AgCl) by computer can be placed into awater solution that contains one molar concentration of electrolyteNaNO₃, one millimolar concentration of electrodeposition precursorCuSO₄, and one millimolar concentration of electrodeposition precursorAgNO₃. The computer can be instructed to change the potential of oneelectrode from +0.5 V to −0.3 V and then to hold the potential at thatvalue for 0.01 seconds. The computer can then be instructed to changethe potential from −0.3 V to +0.5 V and then to hold the potential atthat value for 0.09 seconds. The computer can be instructed to imposethis potential/time “pulse” waveform multiple times (“cycles”) onto theelectrode until the electrode is covered by a sample of suitablethickness. The computer can then be instructed to perform identical (ordifferent) waveforms onto a second electrode until another deposit isformed.

Preferably, the pulse duration is of sufficient time to deposit most ofthe precursors in the diffusion layer but not of sufficient time todeposit precursors which encounter the surface through mass transportprocesses. Likewise, the rest duration is preferably of sufficient timeto allow the precursor dissolved in the bulk solution (i.e. away fromthe electrode) to repopulate the diffusion layer. Typical values forthese durations are pulse durations of less than 100 seconds, morepreferably pulse durations of less than 10 seconds, more preferablypulse durations of less than 1 second, more preferably pulse durationsof less than 0.1 seconds, more preferably pulse durations of less than0.01 seconds, more preferably pulse durations of less than 0.001seconds, more preferably pulse durations of less than 0.0001 seconds,more preferably pulse durations of less than 0.00001 seconds, or mostpreferably pulse durations of less than 0.000001 seconds. Typical valuesfor the rest durations include rest durations of less than 100 seconds,more preferably rest durations of less than 10 seconds, more preferablyrest durations of less than 1 second, more preferably rest durations ofless than 0.1 seconds, more preferably rest durations of less than 0.01seconds, more preferably rest durations of less than 0.001 seconds, morepreferably rest durations of less than 0.0001 seconds, more preferablyrest durations of less than 0.00001 seconds, or most preferably restdurations of less than 0.000001 seconds.

In order to prepare deposits of varying atomic composition, the relativeconcentrations of the electrodeposition precursors can be varied afterpreparation of one deposit on one electrode and before preparation of asecond deposit on a second electrode. For instance, a deposit can beformed at one electrode from a 1:1 molar ratio solution of CuSO₄ andAgNO₃ and a deposit of different composition can be formed at a secondelectrode from a 2:1 molar ratio solution of CuSO₄ and AgNO₃.

This invention provides at least two unexpected advantages: the use ofpulse electrodeposition with an array of electrodes minimizes variationsin morphologies observed when preparing multiple samples and allowsready comparison of chemical and physical properties of the samples; andthe use of an array of electrodes with pulse electrodeposition allowsthe electrochemical variables used in the pulse electrodeposition suchas starting potential, pulse potential, pulse duration, rest time,temperature, electrolyte concentration, relative and combinedconcentrations of the precursors, and number of cycles to be varied fordifferent deposits in order to affect the physical and chemicalcharacteristics of the deposits.

Processing

After deposition of the materials on the electrode array an additionalstep may be employed to further process the materials for analysis. Theprocessing step may occur while the material array is still withincontact of the solutions or other components of the deposition step.Processing may include exposure of the array of materials to gaseous,liquid, or solid reactants, controlled heating or cooling of the arrayof materials, and treatment of the array of materials withelectromagnetic radiation of wavelength between 10⁻¹⁶ m and 10⁻⁸ m.

Analysis

This invention also relates the use of arrays of electrodes as describedto analyze materials comprising two or more elements. The array ofelectrodes may be used to synthesize the array of materials, but thearray need not be used to synthesize the materials. Preferably, thearray of electrodes is used to synthesize and analyze the array ofmaterials. Methods used to analyze the array of materials may or may notcomprise combinations of one or more methods including, but not limitedto electrochemical analysis of materials via the electrodes contained inthe array or via some external electrode assembly. Other analysistechniques include electrochemical analysis of the materials usingelectrochemical methods including but not limited to potentiometry,coulometry, voltammetry, and polarography; analysis of the materials viaoptical methods including but not limited to infrared, Raman, electronicabsorption, fluorescence, phosphorescence, and chemiluminescencespectroscopies, atomic spectroscopy, emission spectroscopy based onplasma, arc, and spark atomization, nephelometry, turbidity,refractometry, polarimetry, rotatory dispersion, and circular dichroism;analysis of the materials via x-ray spectroscopies including, but notlimited to, x-ray fluorescence, absorption and diffractionspectroscopies; analysis of the materials via electron spectroscopicmethods including, but not limited to, x-ray photoelectron spectroscopy,ultraviolet photoelectric spectroscopy, Auger spectroscopy, ionneutralization spectroscopy, electron impact spectroscopy, and penningionization spectroscopy; analysis of the materials via nuclear magneticresonance methods including, but not limited to, nuclear magneticresonance spectroscopy and electron spin resonance spectroscopy; andanalysis of the materials via other methods including, but not limitedto, radiochemical methods, mass spectrometry, conductometric methods,thermal methods, and chromotographic separations.

Once prepared, the array of samples can be examined for chemicalreactivity (such as corrosion, poisoning by carbon monoxide, sulfur orother catalyst poisons, or other types of chemical reactions),electrochemical activity, catalytic or electrocatalytic activity towarda solution-bound species, conductivity, morphology, surface area,surface composition, bulk composition, thickness, presence ofinterfacial layers (such as metal oxide films or mixed metal oxidefilms), thickness of such layers, and other properties of relevance tothe desired application, including the dependence of the propertiesabove on temperature, pressure and solution composition. In order toexamine the samples for properties which involve redox events orcharging (e.g. non-Faradaic) events, the array is “strobed” with aninterrogating potential or current or combination. While each sample isinterrogated, the resulting potential or current or combination of thetwo is monitored. A preferred embodiment is where the potential orcurrent or combination of the two is strobed across the array of samplesat a rate of more than ten samples per one hundred seconds, or morepreferably at a rate of more than ten sample per ten seconds, or morepreferably at a rate of more than ten samples per one second, or morepreferably at a rate of more than ten samples per 0.1 second, or morepreferably at a rate of more than ten samples per 0.01 seconds, or morepreferably at a rate of more than ten samples per 0.001 seconds, or morepreferably at a rate of more than ten samples per 0.0001 seconds, ormost preferably at a rate of more than ten samples per 0.00001 seconds.In a typical experiment, the potentiostat is directed to impose aconstant current across one sample while the potential of the sample,referenced to a reference electrode in solution and charge balanced by acounter electrode in solution, is monitored. The sample is then returnedto near zero current and the potentiostat is directed to perform asimilar “polarization” experiment on another sample. This interrogationis continued until the performances of all of the desired samples havebeen examined. In another typical experiment, the potentiostat isdirected to apply constant or varying potentials across a set of samplesin a high-speed serial manner while the resultant currents are likewisemonitored and recorded.

Morphology

The control of the surface morphologies of the deposited compositions iscritical to any meaningful analysis or screening of the compositions fordesired properties or characteristics. Variations in morphology mayoccur at different electrodes when conditions are not constant for eachdeposition step for different electrodes. Such variations in morphologyintroduce considerable difficulty when comparing the physical andchemical properties of the deposits. A first composition with a firstmorphology may show exceptional properties when analyzed or screened yetthe first composition may still have inferior properties orcharacteristics compared to other compositions on the electrode arraywith a different morphology then the first morphology. Likewise, anarray of samples that all possess rough morphologies may exhibitanomalous characteristics in total when compared to bulk or smoothsamples. By controlling the morphologies of all of the compositions onthe array the screening or analyzing process more readily identifies thecomposition with the desired properties or characteristics.

Another way of controlling the morphology of the depositions employs theuse of a test macroelectrode array. The macroelectrode array allows theelectrochemistry for a class of compositions to be explored with feweryet larger electrodes. This provides a convenient amount of deposit tobe prepared, enabling easy and rapid analysis of the chemical andphysical characteristics of the deposit, including surface morphology,thickness, composition, compositional distribution (i.e. homogeneity),etc. This information can then be used to determine experimentalconditions used in preparing an array of samples onto an array ofelectrodes.

Pulse Electrodeposition

Pulse electrodeposition techniques can be used in combination witharrays of electrodes to prepare libraries of samples that have similaror dissimilar surface morphologies. For example, an array of electrodescan be placed into a solution containing electrodeposition precursors.One or more electrodes can then be placed under potential control insuch a way that the electrodeposition precursors are reacted to formdeposits on them. Such reaction can occur by reducing the Fermi level(or “potential”) of the electrodes until electrons pass from theprecursors into the electrodes, where that electron transfer isaccompanied by deposition of the product deposit.

In a typical situation, such “reductive deposition” as described aboveresults in deposits with widely varying morphologies that depend onprecursor concentration, movement of the solution, temperature, andpotential.

In the present invention, the morphologies of the deposits that areprepared onto the array of electrodes are controlled to be similar ordissimilar by using pulse electrodeposition. This method is particularlysuited to preparation of samples with fairly smooth surfaces. Thismethod involves pulsing the potential to regions where the precursorsare reduced, followed by returning to a potential region where littlereaction occurs. FIG. 1 shows the potential vs time plot typical of apulse electrodeposition synthetic experiment. The starting potential,pulse potential, pulse duration and rest time between pulses can becontrolled to provide the morphology required.

For example, an array of 1024 electrodes that are individuallycontrolled and held at +0.5 volts (V) (vs. the standard electrochemicalreference redox couple of Ag/AgCl) by computer can be placed into awater solution that contains one molar concentration of electrolyteNaNO₃, one millimolar concentration of electrodeposition precursorCuSO₄, and one millimolar concentration of electrodeposition precursorAgNO₃. The computer can be instructed to change the potential of oneelectrode from +0.5 V to −0.3 V and then to hold the potential at thatvalue for 0.01 seconds. The computer can then be instructed to changethe potential from −0.3 V to +0.5 V and then to hold the potential atthat value for 0.09 seconds. The computer can be instructed to imposethis potential/time “pulse” waveform multiple times (“cycles”) onto theelectrode until the electrode is covered by a sample of suitablethickness. The computer can then be instructed to perform identical (ordifferent) waveforms onto a second electrode until another deposit isformed.

In order to prepare deposits of varying atomic composition, the relativeconcentrations of the electrodeposition precursors can be varied afterpreparation of one deposit on one electrode and before preparation of asecond deposit on a second electrode. For instance, a deposit can beformed at one electrode from a 1:1 molar ratio solution of CuSO₄ andAgNO₃ and a deposit of different composition can be formed at a secondelectrode from a 2:1 molar ratio solution of CuSO₄ and AgNO₃.

This invention provides at least two unexpected advantages: the use ofpulse electrodeposition with an array of electrodes minimizes variationsin morphologies observed when preparing multiple samples and allowsready comparison of chemical and physical properties of the samples; andthe use of an array of electrodes with pulse electrodeposition allowsthe electrochemical variables used in the pulse electrodeposition suchas starting potential, pulse potential, pulse duration, rest time,temperature, electrolyte concentration, relative and combinedconcentrations of the precursors, and number of cycles to be varied fordifferent deposits in order to learn how they affect the physical andchemical characteristics of said deposits.

Potential Control to Avoid Exchange Reactions

When electrodepositing samples that contain two or more metals fromsolutions that contain two or more electrodeposition precursors, thedeposits react with the precursors to degrade or change the depositsurface composition, surface morphology, and other characteristics. Thisreaction, illustrated in FIG. 6 a, occurs between deposited atoms 601 &602 and the precursor(s) 603 & 604. The deposited type of atoms thatpossess less positive redox potentials 601 than the Fermi levels of theprecursor(s) 604 are involved in an exchange reaction. The depositedtypes of atoms that posses more positive redox potentials 602 remainunaffected by the associated precursor 604. The type of atoms with lesspositive redox potentials 601 exchange with the precursor(s) 604 whichresults in a deposit having only one type of atom at the surface of thedeposit as shown in FIG. 6 b. For example, a deposit of copper andsilver placed in a solution containing copper salt and silver salt willspontaneously and preferentially reduce silver from the salt to givesilver metal at the deposit while dissolving copper salt from theelectrode. This event interferes with clear and efficient control overelectrodeposition used to prepare the composition and morphology sample.

The present invention describes a method whereby the array of electrodesis held under potential control by the potentiostatic device at all timeduring the synthesis, processing and analysis of an array of samplesdeposited onto the array of electrodes. As shown in FIG. 7, theelectrodes 701 with deposits of materials are supplied with a potentialto control the exchange reaction of precursor 702, while the addressedelectrode 703 has the desired deposition reaction 704 taking place. Thispotential control avoids uncontrolled reaction between the deposits andthe precursors in solution.

An unexpected benefit of this invention include the ability to preparearrays of materials onto arrays of electrodes where the materials havecompositions and morphologies that are a result of the electrodepositionmethod employed and not of uncontrolled side reactions.

Overpotential Electrodeposition

Nucleation of electrodeposits occurs by reduction of anelectrodeposition precursor to give a reaction product and attachment ofthe product to the electrode surface. Growth of electrodeposits occursby reduction of an electrodeposition precursor to give a reactionproduct and attachment of the product to the previously depositedmaterial.

As shown in FIGS. 5 & 5 a, if the electrode potential 506 used at anelectrode 501 to reduce the electrodeposition precursor is the same orsimilar to the standard reduction potential 507 of that precursor, mostelectrodeposition occurs by growth onto fewer nucleation sites. Deposits504 prepared in this manner typically exhibit rough surfacemorphologies.

As shown in FIG. 5 b, if the electrode potential 505 used at anelectrode 502 to reduce the electrodeposition precursor is much lessthan the standard reduction potential 507 of that precursor, mostelectrodeposition occurs by nucleation at the nearest surface. Deposits503 prepared in this manner typically exhibit smooth surfacemorphologies.

This invention describes a method that is comprised of the use of anarray of electrodes and a method of electrodeposition whereby thepotential of each electrode is controlled so that preparation ofdeposits with identical, similar, or different compositions and similaror dissimilar surface morphologies is enabled. An unexpected benefit ofthis method is that ready comparison of the physical and chemicalproperties of the deposits is possible without adjustment for real orperceived variations in morphology between deposits.

Use of Kinetically Sluggish Precursors

When electrodepositing samples that contain two or more metals fromsolutions that contain two or more electrodeposition precursors, thedeposits react with the precursors to degrade or change the depositsurface composition, surface morphology, and other characteristics. Thisreaction, illustrated in FIGS. 6 a & 6 b, occurs between deposited typesof atoms that possess less positive redox potentials than the Fermilevels of the precursor(s). For example, a deposit of platinum andsilver placed in a solution containing platinum salt and silver saltwill spontaneously and preferentially reduce silver from the salt togive silver metal at the deposit while dissolving platinum salt from theelectrode. This event interferes with clear and efficient control overelectrodeposition used to prepare the composition and morphology sample.

The present invention describes a method whereby kinetically sluggishredox species are employed as the electrodeposition precursors. The useof such species for this application is both novel and of great utility.In this method, the spontaneous reaction between some or all of thedeposited material and the precursors in solution is significantlyslowed by employing kinetically sluggish precursors. Thus,electrochemical reaction between the deposit and a precursor occurs onlywhen the reaction is initiated by a deliberately-applied overpotentialat the deposit.

An unexpected benefit of this invention include the ability to nowprepare arrays of samples onto arrays of electrodes where the sampleshave compositions and morphologies that are a result of theelectrodeposition method employed and not of uncontrolled sidereactions. Another unexpected benefit of this invention include theability to now prepare arrays of samples onto arrays of electrodes wherethe samples can remain in solutions containing the precursors withoutdegradation. This ability enables ready preparation of samples of morethan one composition.

Passivating Layers

An additional control technique is the use of passivating layers toprevent deposits from reacting with solution phase components orprecursors. Such passivating layers are made of materials that can beelectrochemically grown and removed. Thus, the use of potential controlcan effect the availability (or lack of it) of a given layer.

As shown in FIGS. 8 a-f, the electrodes 801, 802 & 803 are immersed in asolution containing the passivating agent(s) and the desired precursors.The electrodes 801-803 are first passivated by the passivating agent(s)to form passivating layers 804. The electrode 802 is then biased toremove the passivating layer 804. The deposition of the desiredcomposition 805 then occurs at electrode 802. The electrode 802 is thenpassivated again with a passivating layer 804 where the desiredcomposition 805 is also protected by the passivating layer 804. The nextelectrode 803 to be deposited is then biased to remove the passivatinglayer 804 and addressed for deposition. The steps of biasing anelectrode, depositing a desired composition and passivating theelectrode and composition are repeated for the entire array.

Examples of passivating agents are self-assembled monolayers, oxides andelectropolymerized films. Self-assembled monolayers attach to everyelectrode and passivate the entire array of electrodes. A singlepassivating agent or combinations of passivating agents may be used.These agents can include thiols, pyridine and its derivatives, amine orthiol terminated dendrimers, etc. The passivation layer is removed fromthe electrode where the next deposition is desired via electrochemicalhydrogen or oxygen evolution via a chemical desorption step.

For metals that form stable, blocking (i.e. relatively passive) oxidefilms, the oxide is electrochemically formed via oxidation of thedeposit in aqueous solution. Then, that deposit is not available forfurther deposition chemistry until the oxide has been purposefullyreduced. The deposition solution conditions are typically oxidizing, sospontaneous and uncontrolled reduction of these oxide films in suchsolutions is not likely.

An electropolymerized films is used to block the surface toward anyfurther chemistry until the layer is removed (e.g. electrochemically orvia laser irradiation, photochemically or via ablation). Oxidation ofphenol is a preferred example, because it is a self-healing passivatingfilm (if a spot is left uncoated, phenol gets in and is oxidized toblock that spot). Vigorous gas evolution is employed to remove thepassivating layer. Another embodiment is to drain the flow cell, thenirradiate a given spot with UV (through a quartz widow) and ablate thefilm off of the metal surface.

Use of an Array of Electrodes to Prepare, Process, and Assay an Array ofSamples

An array of electrodes that is controlled by a multiplexed potentiostatinstrument can be placed into solution. Typically, the array ofelectrodes is comprised of fifty or more electrodes, or more preferably100 or more electrodes, or more preferably 1000 or more electrodes, ormore preferably 10,000 or more electrodes, or most preferably 100,000 ormore electrodes.

The solution is comprised of one or more species that are affected byelectrochemistry. For example, a species can be affected by removing oneor more electrons from it, by adding one or more electrons to it, byapplying a potential across it, by changing the conditions of thesolution near the electrode such that these conditions lead to thedeposition of the species from solution, or by similarly affectingsolvent or other species that interact in some manner with the species.

Specifically, one manner in which a species can be affected involvesusing an electrode to remove one or more electrons from a species or addone or more electrons to a species so that the species precipitates fromsolution onto the electrode. This event is termed electrodeposition.

As an example of an experimental context suitable for electrodeposition,the array could be immersed in a solution that contains 1 mM of a metalsalt such as CuSO₄. Typically, the solution will also contain 1 M of anelectrolyte salt such as NaNO₃, although this is not necessary.

As another example of an experimental context suitable forelectrodeposition, the array could be immersed in a solution thatcontains 1 mM of a metal salt such as CuSO₄ and 1 mM of another metalsalt such as AgNO₃. Other contexts suitable for electrodepositioninclude those where the number of metal salts is more than two, thosewhere the relative concentrations of the metal salts are varied from 1 Mto 0.000001 mM, those where the electrolyte concentration is varied from5 M to 0.001 mM, and those where other experimental variables (such astemperature, movement of the solution, type of solvent, etc.) arecontrolled to provide additional conditions for electrodeposition.

One preferred experimental context is to immerse an array of 1024electrodes into a water solution-containing 1 M NaNO₃, 0.5 mM CuSO₄, and0.5 mM AgNO₃. The solution is contained in a “flow cell” thatmechanically holds the array in place and provides inlet and outlettubes to flow the solution across the array of electrodes. The flow cellalso contains mechanical devices (holes) that allow inclusion of acounter electrode and a reference electrode. Neither electrode isincluded as a part of the array. Alternately, the reference electrodecan be included as a part of the array. The counter electrode isfabricated to be of high surface area and positioned so that theelectrochemical events occur similarly across the array. One or moreelectrodes in the array are used in conjunction with the counter andreference electrodes to perform electrochemistry at the one or moreelectrodes in the array. The electrochemistry that is performed involvespulse electrodeposition, whereby the potential of one or more electrodesin the array is reduced to a less positive value than that of one ormore precursors in solution to remove one or more electrons from one ormore precursors, and then the potential is returned to a more positivevalue where electrons are not removed from at least one precursor. Thispulse is repeated from two to several million times at one electrodeuntil the desired amount of deposit has been formed. The relativeconcentrations of the precursors in solution are then changed by a smallincrement to 0.49 mM CuSO₄ and 0.51 mM AgNO₃ and the pulsedelectrochemical process is repeated at a different electrode. Thisincremental change in relative precursor concentration and subsequentpulsed electrodeposition is repeated until all concentrations ofinterest have been used to prepare discrete samples on each electrode ofthe array of 1024 electrodes. Those samples can then be treated using anumber of condensed phase and gas phase treatments that include, forexample, exposure to carbon monoxide gas. Alternately, those samples canbe analyzed for desired activity without treatment. The analysis ofthose samples can occur by electrochemical interrogation of each sample,of a group of samples, or of all samples using the array of electrodesthat supports the samples. This ability to individually address thesamples during analysis is an unexpected benefit of preparing thesamples onto an array of electrodes. For example, the array ofsample-supporting electrodes can be immersed in an 80° C. water solutionthat contains oversaturated hydrogen gas, and the potential of anindividual electrode that support a sample of interest can be held at0.7 volts positive of the Ag/AgCl redox couple while the current that ispassed through the electrode is monitored. That sample-supportingelectrode can then be returned to open circuit or another potential, anda similar analysis can be performed at a second sample-supportingelectrode. This process can be continued until all of the samples ofinterest have been examined. The information that is obtained canindicate the array of activities that the array of samples show foroxidizing hydrogen. That array of activities can be examined manually orby algorithmic computer search to indicate samples that show exceptionalor unexpected activities for oxidation of hydrogen. Samples which showexceptionally high activities for oxidation of hydrogen can then beemployed to indicate compositions of matter that can be useful fordevices that are used to oxidize hydrogen, such as fuel cells andelectrolytic cells.

Abstract: Methods and apparatus employ the use of arrays of two or moreelectronically-discrete electrodes to facilitate high-throughputpreparation and testing of materials comprising two or more elements.High rates of deposition, synthesis and/or analysis of materials areachieved with the use of arrays of electrodes whereby desired materialsare developed. The high rate synthesis and/or analysis of an array ofmaterials uses deposition control techniques in conjunction with theelectrode array to develop a meaningful array of materials and toanalyze the materials for a desired characteristics to develop one ormore materials with the desired characteristics. The use of an array ofelectrodes is a technique for high-throughput development of materialshaving scientific or economic advantages.

EXAMPLE 1 Synthesis and Screening of Anode Catalysts for Fuel Cells

The fabrication and testing of binary phase space at a resolution of oneatomic percent (e.g. 100% Pt-0% Ru, 99% Pt-1% Ru . . . 1% Pt-99% Ru,100% Ru) requires the fabrication and evaluation of 101 different alloycombinations. The addition of another component increases the number ofdifferent ternary compositions to 10,100 and the addition of a fourthcomponent increasing the number to over 1,000,000. The testing andfabrication of such large numbers of materials using traditional methodsis unwieldy as to make it experimentally impossible. Consequently,researchers have had to rely on chemical intuition, and scantexperimental results, to lead them in the direction of improved catalystsystems. Combinatorial catalyst electrodeposition directly onto anelectrode array allows rapid preparation and testing of samples.

Binary compositions can be electrodeposited from aqueous solutionscontaining metal complexes of the desired metals, in particular PtCl₄ ²⁻and RuCl₆ ³⁻. These dilute solutions have molarities in the range of1-10 mM. The use of high valence metal anions allows for use of mutuallymiscible deposition solutions. Suitable experimental conditions for theco-deposition of Pt—Ru compositions on bulk electrochemical electrodes(standard electrodes for electrochemical evaluation) can be initiallydetermined using bulk electrodes. These conditions can then be used fordeposition of compositions upon the electrode array.

Overpotential deposition is the method of choice. The fabrication may beautomated by computer control of the solution composition in thedeposition cell, and, since electrodeposition are carried out at thesame overpotentials, morphology variations will be minimized. Thissimilarity in particle size, morphology, and surface area is essentialfor the comparison of catalytic reactivities of different compositions,simplifying data analysis.

Procedures to be used to electropotentially evaluate theelectrocatalysts for methanol and CO oxidation involve two differenttesting protocols. The reactivity of the electrocatalystselectropotentially in an essentially linear (but extremely rapid andhighly automated) fashion is analyzed. The primary method by which thereactivity of the many alloys is examined will be based on directelectrochemical measurement of the electrocatalytic reaction rate. Thisis done using traditional electrochemical approaches. Previous studieson the activity of Pt—Sn and Pt—Ru alloys toward CO oxidation, used COstripping voltametry (i.e. predosing the solution with CO, followed byoxidation in a clean electrolyte) may be used as a means ofdetermination of catalytic activity towards CO oxidation. The testingprocedure may be modified to measure the current that results from thecathode at specific voltages as the level of carbon monoxide in solutionis increased. Test procedures may also evaluate the ability of thecatalyst to reactivate itself after exposure to CO.

Evaluation of a 1024 microelectrode array may take place in just over 17minutes for a given set of conditions. Note that, while theseexperiments are not strictly combinatorial (i.e., there are no parallelsteps in the evaluation, except that same solution conditions are usedfor each electrode) the development of a highly automated testinginstrument achieves the same end goal, namely, the ability to prepareand test extremely large numbers of electrocatalyst compositions inshort time periods.

Direct electrochemical potentiometric experiments on the electrode arraysequentially and rapidly addresses the individual electrodes with both apotential step and sweep under either hydrostatic or hydrodynamicconditions. The current response of the materials indicates the kineticsfor the reactions of interest, e.g. H₂ and MeOH oxidation.

The oxidation of MeOH to CO₂ on pure platinum occurs at an overpotential0.5-0.6V above the thermodynamic potential of (+0.043). Consequently,after preparation of an array of materials of varying composition, thearray is immersed into a flow cell containing methanol. The entire arrayof electrodes are biased simultaneously at the overpotential formethanol oxidation (0.7V). The methanol electrooxidation currentdensities measured will indicate lead compositions worthy of furtherdevelopment and analysis.

EXAMPLE 2 Synthesis and Screening of Cathode Catalysts for Fuel Cells

The combinatorial electrochemistry techniques described above are usedto synthesize and characterize known and new catalysts for use ascathode catalysts in fuel cells. The primary desired characteristic inthis example is a high rate of reduction of oxygen. A secondary desiredcharacteristic is a high selectivity for oxygen over other species suchas methanol.

The aqueous salt solutions used, for example, may include H₂PtCl₄(tetrachloroplatinic acid) and Cr₂(SO₄)₃ (chromic sulfate) with NaNO₃electrolyte. Electrodeposition takes place in a typical three electrodeconfiguration comprised of a reference electrode, a counter electrodeand the working electrode array. Electrodeposition is carried out undera variety of concentrations, flow rates, currents, potentials, andtemperatures to achieve the most smooth and reproducible surfaces.Typically, overpotential deposition, high frequency pulse deposition,and protecting the deposits with passivating layers such as selfassembled monolayers, electrooxidizable polymers, and metal oxidecoatings allows preparation of smooth, well-controlled samples.

Reduction of Pt²⁺ occurs at ca. −0.2 V and reduction of Cr³⁺ occurs atca. +0.75 V. Unprotected deposits which include Pt that are allowed toreturn to open circuit will experience reaction between Pt and Cr³⁺ togive Pt²⁺ (as a dissolved salt) and Cr (as a deposited metal). Thus, thepotentials of “inactive” electrodes are controlled to be more positivethan +0.75 V.

Rough morphology may be mitigated further by the use of high valencemetal anions and the incorporation of complexing agents, such assulfamate urea or substituted urea derivatives, formamide or other amineadditives in the deposition baths. These can increase the polarizationof chromium and enhance simultaneous codeposition of both the componentsof the bath. However, the effects of such additives on the catalysts mayinterfere with the overall activity of the alloys.

For analysis of the deposited samples, in order to establish a pertinentexperimental protocol, catalytic activity can be ascertained first forbulk samples. The primary method of analysis is direct electrochemicalmeasurement of the electrocatalytic reaction rate. This can be doneusing traditional electrochemical approaches. For instance, CO strippingvoltammetry (i.e. pre-dosing the solution with MeOH, followed byoxidation in a clean electrolyte) can be used as a means ofdetermination of catalytic activity towards MeOH activity. This testingprocedure is modified to measure the current that results from thecathode at specific voltages as the level of methanol in solution isincreased. Test procedures can also be included to evaluate the abilityof the catalyst to reactivate itself after exposure to transient MeOH.

Characterization of oxygen reduction catalytic activity can beaccomplished via half-cell testing. This method requires the use of anoxygen saturated solution. The compositions are tested at 1.0-0.2 V vs.a reference hydrogen electrode. This test results in the determinationof polarization curves for the catalytic oxygen reduction of the variouscompositions (potential (V) vs. specific current density (A/cm²)).

Resistance to methanol and, by association, carbon monoxide, is measuredby introducing methanol in 5 mM increments to the test solution. Thisstep provides an absolute measure of both the selectivity of thecatalyst for oxygen reduction and the resistance of the catalyst tomethanol or CO poisoning.

EXAMPLE 3 Deposition from Solutions of Metal Precursor Complexes

Deposition from solutions containing the metals as precursor compoundsis a flexible approach, especially with regard to depositing more thanone element simultaneously. In the simplest embodiment of this approach,the solution contains a mixture of metal alkoxides. Deposition occursvia an electrochemical process.

For example, if the metals are present as alkoxide precursors anddissolved in a mixed alcohol/water solvent, the deposition could beeffected by creating oxidizing conditions at a particular electrode inthe array of electrodes. These oxidizing conditions can produce protons,thereby driving the decomposition of the metal alkoxide precursors,precipitating the metal oxide product, and depositing a mixed metaloxide on the electrode. In this simplest case, the deposit comprises ametal oxide with the metal composition being defined by the relativeamounts of the metal alkoxide precursors in the original solution.

Control of the metal oxide composition may be achieved by varying thecomposition of the solution from which the deposition occurs. Thus, thisapproach is controllable and reproducible to produce varied metal oxidedeposits.

Deposition of metals in various host matrices is achieved by addition ofthe appropriate host compounds. For example, to deposit a metalsilicate, a silicon precursor compound such as tetraethoxysilane isadded to the solution. Metal aluminates may be deposited from solutionscontaining aluminum alkoxide reagents, etc.

Several other types of precursors can also be used, including oxalates,acetates, and properly-tuned crown ether complexes. In these cases,dissociation of the complexing agent from the metal center can be drivenby redox decomposition of the complex. For the oxalates in particular,this leads to clean production of the “bare” (i.e. solvated) metal ionplus CO₂. In the presence of alkoxides such as those discussed above,these metal ions become trapped within the silica or alumina (or othermaterial) matrix. This point is important because it suggests thatseveral different metals can be codeposited from a variety ofcomplexation environments, which will allow for unexpected flexibilityin the design of the deposition conditions.

The compositions of typical target phosphors can derive fromcombinations of the following materials: type A elements (Si, Al, P, B,Mg, Ge, Sn), type B elements (Ba, Sr, Ca, Y, Gd, K, Na), activators (Eu,Th, Ce), and sensitizers (any other rare earth metal). Host matricesprepared from combinations of the type A and type B elements can then bedoped with small amounts (1-15%) of the activators and smaller amounts(0.001-1%) of the sensitizers. Given this method for materialssynthesis, an extremely large number of possible phosphors isenvisioned. An example is the phosphor defined by Si as the type Aelements, a combination of Y and Ba as the type B element, with Ce addedas an activator and Nd added as a sensitizer. For this elementalcomposition, samples are electrochemically prepared that contain thetype A element compositionally varied in 5% increments, giving 20possible combinations. For each of these 20 combinations, theconcentration of type B elements in the samples is likewise varied in 5%relative w/w increments, giving a total of about 400 compositions. Foreach of these, the activator may be varied from 1-15% in 1% increments,giving a total of 6000 compositions. Finally, for each of these, thesensitizer may be varied from 0.001% to 1% in factors of 2 on alogarithmic scale (i.e. 0.001%, 0.003%, 0.01%, etc.) for a total ofseven additional variations. This brings the total number ofcompositions to 42,000 for a fairly simple phosphor material.

For this example, alkoxide complexes for all of the precursor metals canbe used. Specifically, these are Y(OEt)₃, Ba(OEt)₂, Si(OEt)₄ andEu(OEt)₃. Deposition is effected in a solution containing 95-99% ethanoland 5-1% H₂O. It is necessary to keep the water concentration low,because the presence of large amounts of water in the solution can drivepremature hydrolysis of the sol-gel precursors, leading to bulk phaseprecipitation of the material. In this particular case, sol-geldeposition is achieved by applying a more positive electrode potential.This oxidizes water and leads to hydrolysis of the alkoxide complexes,leading to deposition of a metal silicate containing the metals thatwere present.

Key variables for the deposition step include: a) the temperature, whichinfluences the rates of the hydrolysis and deposition processes, b) theapplied potential, which influences the rate of water oxidation andtherefore the rate of generation of protons, c) the manner in which thepositive potential is applied, either galvanostatically orpotentiostatically, which influences the time dependence of the rate ofproton generation, d) the time for the deposition, which influences theamount of material deposited, e) using solution flow to replenishreagents near the electrodes, and f) the structure of the alkoxide orother precursor complex, which affects the stability and hydrolysis rateof the complex. Exploration of these variables may be done at bothconventional bulk electrodes and on an array of electrodes.

A fairly wide range of other solution/deposition conditions isenvisioned for metal oxide deposition from solution. For example, metalsalts can be used in aqueous solutions as precursors formetal-containing thin films of oxides or other phases. In these cases,deposition is driven by interfacial pH changes and/or by changes in theredox state of the metals (i.e. to generate an oxidation state that isnot soluble under the solution conditions). This technique may be usedto deposit PbO₂ films doped with a wide variety of second metals,including As and Bi. Adapting this technique to allow co-deposition ofmany different, mixed composition metal phases, including oxides,silicates and aluminates, using precursors that are modestly stable inaqueous solutions, at arrays of electrodes is an innovative step.

For example, for aluminates, one can start with solutions containingNaAlO₂-like species. Execution of this approach requires identificationof suitable precursors and exploration of their regions of mutualstability in aqueous solutions and conditions under which they can beco-deposited.

One more example is of preparation of a simple metal phosphate phosphor.In a typical experimental protocol, the electrode array is housed in aflow cell, so that it can be exposed to a solution of controlled, butvariable, composition. For example, for deposition of a europium(II)-doped BaPO₄ material a solution containing the appropriate mixtureof the rare earth and alkaline earth ions (typically as nitrate salts),plus a source of phosphate (e.g. H₃PO₄), is introduced into the flowcell. Applying a less postive potential drives water reduction, whichcauses the pH near the electrode surface to high values. This producesPO₄ ³⁻ which drives deposition of the target material. This process canbe repeated under computer control at many different electrode locationsin a given array, but with slight variations in the solution phasecomposition in order to produce a new and slightly different compositionat each electrode. In this way, the entire compositional phase space forthe material is examined in a short time.

EXAMPLE 4 Deposition from Colloidal Solutions

An additional approach to deposition of various oxide, silicate,aluminate, and other materials is through the deposition of combinationsof small colloidal particles containing the various materials that aredesired to be in the deposit. Deposition from colloidal solutions can beachieved by electrochemically driving a flocculation process. In such acase, the material precursors are present as constituents of a colloidalsuspension.

tabilization of such suspensions is typically electrostatic. By controlof the surface charge on the particles and the ionic strength of thesolution it is possible to generate stable suspensions. Surface chargecontrol is achieved either by manipulating the pH of the solution totake advantage of the acid-base character of the colloidal particles orby manipulating the identity, charge and number of adsorbates on theparticles. Ionic strength is used to control the electrostatic repulsionbetween the particles. Low ionic strength gives greater repulsion andmore stability while higher ionic strength gives less repulsion and lessstability.

Given this situation, it is clear that deposition from a colloidalsuspension can be driven by causing any of several changes in thesolution conditions near the electrodes in the array of electrodes. Forexample, a simple case involves the use of colloidal particles that arestabilized by surface charges that derive from acid-base chemistry atthe particle surface. An example is γ-Al₂O₃. This material has apH_(pzc) of 8.5. Thus, at pH values below 8.5, the net surface charge ispositive. Application of reducing potentials at an electrode in anaqueous solution containing such particles drives water reduction. Thisleads to an increase in the pH near the electrode. If this pH change iscarefully controlled, conditions can be created near the surface thatcause the particle surface charges to approach zero. This leads toinstability of the colloidal suspension and results in deposition of theparticles.

More complicated scenarios are needed to describe the deposition of morethan one element using this approach. In these cases, solutionscontaining more than one stable colloidal suspension are prepared. Forexample, stable suspensions of metal oxides and silicates can beprepared. Again, adequate stability can be achieved by using suitablesolutions and ionic strengths so that all the particles in solution beara net positive (or negative) charge and experience significantelectrostatic repulsion due to low ionic strength. Appropriate controlof solution conditions enable colloid mixtures to be compatible (withrespect to flocculation).

An issue in this approach is how the activators and sensitizers can beincorporated into the deposits. New synthetic procedures allow oxidativedecomposition of precursor complexes of the activators and sensitizersduring deposition of the colloidal particles.

As will be understood by ordinary skill in the art, the presentinvention may be embodied in other forms without departing from theessentials thereof. Accordingly, disclosure of the preferred embodimentis intended to be illustrative, but not limiting, of the scope of theinvention which is set forth in the following claims.

1. An apparatus for rapidly applying at least one component of each ofat least two materials to addressable predefined locations on an arrayof electrodes, the apparatus comprising: at least two electrodescontained in said array; and an assembly for applying an electricalpotential or current from a power source to each of the electrodes,wherein the electrodes are addressable through the assembly, and whereinsaid at least one component of said each of said materials isoperatively supplied to the array of electrodes, said electricalpotential or current causing the components of the materials to depositat said addressable predefined location, wherein the deposition iscontrollable.
 2. The apparatus of claim 1, wherein said electrodes arethe addressable predefined locations and wherein the depositings arecontrollable.
 3. The apparatus of claim 2, wherein the assembly includesa controller to control the electrical potential or current for eachsaid electrode.
 4. The apparatus of claim 3, wherein the electrode arrayincludes at least twenty said electrodes.
 5. The apparatus of claim 4,wherein the electrode array includes at least one hundred saidelectrodes.
 6. The apparatus of claim 5, wherein the electrode arrayincludes at least ten thousand said electrodes.
 7. The apparatus ofclaim 5, wherein said assembly includes a reference electrode.
 8. Theapparatus of claim 7, wherein the reference electrode is movable tooperatively adjust depositions.
 9. The apparatus of claim 5, whereinsaid assembly includes a counter electrode.
 10. The apparatus of claim9, wherein the counter electrode is movable to operatively adjustdepositions.
 11. The apparatus of claim 5, wherein the array ofelectrodes includes a highly resistive substrate below said electrodes,a highly conductive material above said highly resistive substrate andaround said electrodes, and a second highly resistive material abovesaid highly conductive material and around said electrodes wherein thesecond highly resistive material does not cover the electrodes.
 12. Theapparatus of claim 11, wherein the array of electrodes is amicroelectrode array.
 13. The apparatus of claim 12, wherein theelectrodes are at most 1 mm in diameter.
 14. The apparatus of claim 13,wherein the electrodes are at most 100 μm in diameter.
 15. The apparatusof claim 5, wherein said at least one component of said each of at leasttwo materials is entrained in a solution when operatively supplied tosaid assembly.
 16. The apparatus of claim 15, wherein at least one ofsaid at least one component is a kinetically sluggish precursor.
 17. Theapparatus of claim 15, wherein said solution includes at least twopassivating agents.
 18. The apparatus of claim 5, wherein the assemblyincludes a flow cell, whereby the at least one component of said each ofthe two at least two materials is supplied to the array of saidelectrodes in varying concentrations of said components.
 19. A method ofrapidly applying at least one component of each of at least twomaterials to addressable predefined locations on an array of electrodes,comprising: applying a potential to at least one first electrode on saidarray of electrodes; depositing at least said at least one component ofeach of the at least two materials onto said at least one firstelectrode; applying a second potential to at least one second electrodeon said array of electrodes; depositing at least said at least onecomponent of each of the at least two materials onto said at least onesecond electrode, said depositing being controllable wherein said atleast one component of each of the at least two materials deposited onthe at least one second electrode varies in composition from the atleast one component of each of the at least two materials deposited onthe at least one first electrode.
 20. A method of rapidly applying atleast one respective component of each of at least two materials toaddressable predefined locations on an array of electrodes, comprising:varying concentrations of the components over time; applying a potentialto at least one first electrode on said array of electrodes; depositingat least the components onto said at least one first electrode to form arespective deposit on each of said at least one first electrode;applying a second potential to at least one second electrode on saidarray of electrodes; depositing at least said components onto said atleast one second electrode to form a respective deposit on each of saidat least one second electrode, wherein said components deposited on theat least one second electrode vary in composition from the componentsdeposited on the at least one first electrode, and wherein thedepositings are controllable.
 21. The method of claim 20, wherein saidcomponents are supplied by a flow cell.
 22. The method of claim 21,wherein said applying include adjusting the potential to operativelysupply an overpotential at said at least one electrode whereby thedepositing of said components leads to uniform morphologies at each saidat least one electrode.
 23. The method of claim 21, wherein saidapplying include controlling the potential at each said at least oneelectrode by pulse electrode position whereby a depositing potential isapplied followed by a resting potential in cycles until said each atleast one electrode is deposited with said respective composition. 24.The method of claim 21, wherein said applying include applying a secondpotential to the other electrodes in the array whereby the electrodesnot being supplied a depositing potential are supplied a holdingpotential to prevent exchange reactions.
 25. The method of claim 21,wherein said components are kinetically sluggish precursors.
 26. Themethod of claim 21, wherein said varying includes moving a reference orcounter electrode.
 27. The method of claim 21, wherein said applying andsaid depositing step is repeated one thousand times in rapid sequencewhereby at least one thousand different compositions are deposited ontothe array of electrodes.
 28. The method of claim 20, wherein saiddepositing include controlling the morphology of each of said deposits.